1. Field of the Invention
The present invention relates to a magnetoresistive element applied to a nonvolatile memory or the like.
2. Related Background Art
Recently, magnetic memory elements for storing information by using a magneto-resistance effect receive attention as high-density, high-response, nonvolatile solid-state storage elements. It has been examined to constitute a RAM (Random Access Memory) by using the magnetic memory element. The magnetic memory element can store information by the magnetization direction of a magnetic layer, and can constitute a nonvolatile memory for semipermanently holding information. Magnetic memory elements are expected to be used as various recording elements such as information storage elements for a portable terminal and card. Especially a magnetic memory element using a spin tunneling magnetoresistance (TMR) effect can utilize a high-output characteristic obtained by the TMR effect. This magnetic memory element also allows high-speed read, and its practical use is expected.
In the magnetic memory element, the minimum unit for storing information is called a magnetic memory cell. The magnetic memory cell generally has a memory layer and reference layer. The reference layer is a magnetic material layer whose magnetization direction is fixed or pinned in a specific direction. The memory layer is a layer for storing information, and is generally a magnetic material layer capable of changing its magnetization direction by externally applying a magnetic field. The logic state of the magnetic memory cell is determined by whether the magnetization direction in the memory layer is parallel to that in the reference layer. If these magnetization directions are parallel to each other because of the MR (MagnetoResistance) effect, the resistance of the magnetic memory cell decreases; if these directions are not parallel, the resistance of the magnetic memory cell increases. The logic state of the magnetic memory cell is determined by measuring its resistivity.
Information is written in the magnetic memory cell by changing the magnetization direction within the memory layer by a magnetic field generated by flowing a current through a conductor. Written information is read out using an absolute detection method of detecting the absolute value of a resistance.
Another memory cell has a memory layer and detection layer. This memory cell employs a differential detection method for read because the magnetization direction of the detection layer is changed and the magnetization direction of the memory layer is detected from a change in resistance.
The magnetic memory cell must shrink in feature size for high integration degrees. Generally in a longitudinal magnetization layer, the spin curls at the film edge due to a demagnetizing field within the film surface along with the miniaturization. The magnetic memory cell cannot stably store magnetic information. To prevent this problem, the present inventor has disclosed in U.S. Pat. No. 6,219,725 an MR element using a magnetic film (perpendicular magnetization film) magnetized perpendicularly to the film surface. The perpendicular magnetization film is free from any curling even upon miniaturization, and is suitable for miniaturization.
A magnetic memory cell using an MR element includes two magnetic layers stacked via a thin nonmagnetic layer (tunnel insulating layer). A magnetic field leaked from one magnetic layer within the magnetic memory cell influences the other magnetic layer. The magnetic field is kept applied even in the absence of an external magnetic field.
FIGS. 20A and 20B show examples of the magnetization direction of a TMR element having a perpendicular magnetization film. A magnetic film 100 having a low coercive force and a magnetic film 200 having a higher coercive force are stacked via a tunnel insulating film 300. In both the examples shown in FIGS. 20A and 20B, the magnetic film 200 is magnetized downward. The magnetic film 100 is magnetized downward in FIG. 20A, and upward in FIG. 20B. Hence, the resistance value of the magnetic memory cell is larger in FIG. 20B than in FIG. 20A.
This state may be considered as a structure using the absolute value detection method in which the magnetic layer 200 is a reference layer (pinned layer), the magnetic layer 10 is a memory layer, xe2x80x9c0xe2x80x9d is recorded as shown in FIG. 20A, and xe2x80x9c1xe2x80x9d is recorded as shown in FIG. 20B. Alternatively, this state may be considered as a structure using the differential detection method in which the magnetic layer 200 is a memory layer, the magnetic layer 10 is a detection layer, and the magnetization is switched from the state shown in FIG. 20A to the state shown in FIG. 20B by an external magnetic field in detection.
FIG. 21A shows the MH curve of this element (graph showing the relationship between the magnetization and the application magnetic field) on the assumption that no magnetic field is leaked from the other magnetic film with a squareness ratio of 1. A magnetic field small enough to keep the magnetization direction unchanged is applied to the magnetic layer 200. Therefore, a curve corresponding to the magnetization direction of the magnetic layer 100 appears. In the absence of a magnetic field leaked from the other magnetic film, i.e., an offset magnetic field, information can be recorded on the memory layer only by applying a magnetic field H1 or H2 equal to a coercive force Hc. Alternatively, the magnetization of the detection layer can be switched. The magnetic field H1 switches the first magnetic film from the upward direction to the downward direction. The magnetic field H2 switches the first magnetic film from the downward direction to the upward direction.
In practice, the other magnetic layer, in this case, the magnetic film 200 applies a downward magnetic field to the magnetic film 100. The MR curve shifts by the offset magnetic field Ho, as shown in FIG. 21B. In this case, the recording magnetic field is H2=Hc+Ho and H1=Hcxe2x88x92Ho. The magnetic field necessary to change the state of FIG. 21B to that of FIG. 21A decreases by Ho. To the contrary, the magnetic field necessary to change the state of FIG. 21A to that of FIG. 21B increases by Ho. This means that a current value flowing through a write line increases. Current consumption may increase, or when the current exceeds the allowable current density of write line wiring, write may fail. In this case, the magnitude of a switching magnetic field changes depending on information recorded on a memory cell. If memory cell information which requires the switching magnetic field H2 is rewritten in recording information in memory cells arrayed in a matrix via two perpendicular write lines, adjacent memory cell information which requires the switching magnetic field H1 is also rewritten. Such erroneous recording operation may occur at a high possibility. If the offset magnetic field Ho becomes larger than the coercive force Hc, as shown in FIG. 21C, only one resistance value can be taken in zero magnetic field. This makes absolute detection difficult.
When the squareness ratio is not 1, a magnetization M in zero magnetic field becomes smaller than a maximum magnetization value Mmax of an antiparallel magnetization state. The resistance value also changes depending on the magnetization magnitude of the low-coercive-force layer. In this case, a readout resistance value difference R2xe2x88x92R1 decreases, degrading the detection sensitivity. This phenomenon occurs even in an offset magnetic field Ho smaller than the coercive force Hc. Note that R1 represents the minimum resistance value in the absence of an external magnetic field; and R2, the maximum resistance value in the absence of an external magnetic field. FIG. 22A shows the resistance value in the presence of the offset magnetic field Ho, and FIG. 22B shows the resistance value in the absence of the offset magnetic field Ho.
For a squareness ratio of not 1, even application of a magnetic field equal in magnitude to the coercive force does not completely saturate the magnetization, as shown in FIG. 22B. A magnetic field which completely saturates magnetization, M Ms, will be called a magnetization saturation magnetic field Hs. When the memory layer completely saturates to be antiparallel to the pinned layer, the resistance value maximizes to a constant value with respect to the magnetic field. That is, the magnetic field which saturates in the resistance value is equal to Hs, as shown in FIG. 22B. For a squareness ratio of 1, the coercive force can be regarded equal to a magnetization switching magnetic field. For a squareness ratio of not 1, the coercive force cannot be regarded equal to this magnetic field. In this case, the magnetization must be switched by applying a magnetic field larger than that having a squareness ratio of 1. In the presence of an offset magnetic field generated by a leaked magnetic field, the difference in the magnitude of a magnetic field applied to switch the magnetization becomes larger between a direction in which the magnetization is easy to switch and a direction in which the magnetization is difficult to switch. If such an element is employed as the memory element of an MRAM, the above-described erroneous operation may occur at a higher possibility. Malfunction may occur when a magnetization switching magnetic field is not controlled in the use of a magnetoresistive element as the memory element of an MRAM.
The above description mainly assumes the absolute value detection method, but similarly applies to the differential detection method. FIG. 23 shows the major loop of the differential detection method.
The above-described problems in the MR element are serious particularly in a magnetoresistive element using a longitudinal magnetization film adopted in a conventional MRAM.
It is an object of the present invention to solve the problem that a static magnetic field from one magnetic layer offsets the switching magnetic field of the other magnetic layer in a magnetoresistive element used as a memory element or the like and the problem that the switching magnetic field increases, and to provide a memory element using this magnetoresistive element and its recording/reproduction method.
To achieve the above object, the present invention provides a magnetoresistive element comprising
a first magnetic layer magnetized perpendicularly to a film plane,
a second magnetic layer which is magnetized perpendicularly to the film surface and has a coercive force higher than a coercive force of the first magnetic layer,
a nonmagnetic layer inserted between the first and second magnetic layers, and
a third magnetic layer which has a coercive force higher than the coercive force of the first magnetic layer and is magnetized antiparallel to the second magnetic layer.